Cyzicus was an ancient town of Mysia in Asia Minor, situated on the shoreward side of the present peninsula of Kapu-Dagh (Arctonnesus), which is said to have been originally an island in the Sea of Marmora, and to have been artificially connected with the mainland in historic times.

It was, according to tradition, occupied by Thessalian settlers at the coming of the Argonauts, and in 756 BC the town was founded by Greeks from Miletus.

Owing to its advantageous position it speedily acquired commercial importance, and the gold staters of Cyzicus were a staple currency in the ancient world till they were superseded by those of Philip of Macedon. During the Peloponnesian War (431-404 BC) Cyzicus was subject to the Athenians and Lacedaemonians alternately, and at the peace of Antalcidas (387 BC), like the other Greek cities in Asia, it was made over to Persia.

The history of the town in Hellenistic times is closely connected with that of the Attalids of Pergamon, with whose extinction it came into direct relations with Rome. Cyzicus was held for the Romans against Mithradates in 74 BC till the siege was raised by Lucullus: the loyalty of the city was rewarded by an extension of territory and other privileges. Still a flourishing centre in Imperial times, the place appears to have been ruined by a series of earthquakes--the last in AD 1063--and the population was transferred to Artaki at least as early as the 13th century, when the peninsula was occupied by the Crusaders.

The site is now known as Bal-Kiz and entirely uninhabited, though under cultivation. The principal extant ruins are the walls, which are traceable for nearly their whole extent, a picturesque amphitheatre intersected by a stream, and the substructures of the temple of Hadrian. Of this magnificent building, sometimes ranked among the seven wonders of the ancient world, thirty-one immense columns still stood erect in 1444. These have since been carried away piecemeal for building purposes.